U.S. patent number 4,005,914 [Application Number 05/603,604] was granted by the patent office on 1977-02-01 for surface coating for machine elements having rubbing surfaces.
This patent grant is currently assigned to Rolls-Royce (1971) Limited. Invention is credited to Paul Newman.
United States Patent |
4,005,914 |
Newman |
February 1, 1977 |
Surface coating for machine elements having rubbing surfaces
Abstract
In a foil gas bearing or other similar machine element the
relatively rigid shaft is coated with a glaze-forming oxide layer
between 0.003 and 0.020 ins. thick, while the relatively thin foils
are coated with a layer of a compound comprising cobalt and
chromium carbide to a depth of up to 0.003 ins., the surface layer
of which is oxidized to a depth of 0.0001 ins. to 0.0005 ins.
Inventors: |
Newman; Paul (Bristol,
EN) |
Assignee: |
Rolls-Royce (1971) Limited
(EN)
|
Family
ID: |
10389912 |
Appl.
No.: |
05/603,604 |
Filed: |
August 11, 1975 |
Foreign Application Priority Data
|
|
|
|
|
Aug 20, 1974 [UK] |
|
|
36637/74 |
|
Current U.S.
Class: |
384/103 |
Current CPC
Class: |
F16C
17/024 (20130101); F16C 2206/42 (20130101); F16C
33/043 (20130101); F16C 2206/82 (20130101) |
Current International
Class: |
F16C
17/00 (20060101); F16C 17/12 (20060101); F16C
035/00 () |
Field of
Search: |
;308/5,9,DIG.1,DIG.7,238,37,121,122,160,241 ;29/195T |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Peters, Jr.; Joseph F.
Assistant Examiner: Church; Gene A.
Attorney, Agent or Firm: Stevens, Davis, Miller &
Mosher
Claims
I claim:
1. A machine element comprising a relatively rigid component and a
relatively flexible component adapted for relative rotation and
wherein the relatively rigid component is provided with a surface
layer of a glaze-forming oxide between 0.003 ins. and 0.020 ins.
thick, and the relatively flexible component is provided with a
surface layer of a glaze-forming oxide between 0.0001 ins. and
0.0005 ins. thick.
2. A machine element according to claim 1 and wherein the oxide
layer in the relatively rigid member is between 0.003 ins. and
0.007 ins. thick.
3. A machine element according to claim 1 and wherein the
glaze-forming oxide on the relatively rigid component is an oxide
of Cobalt.
4. A machine element according to claim 1 and wherein the
glaze-forming oxide on the relatively rigid component is an oxide
of Chromium.
5. A machine element according to claim 1 and wherein the
relatively flexible component is coated with a compound consisting
of cobalt and chromium carbide, the surface layer of which is
oxidized to provide the glaze-forming oxide layer.
6. A machine element according to claim 5 and wherein said compound
consists of 75% cobalt and 25% chromium carbide by volume.
7. A machine element according to claim 1 and wherein the machine
element is a foil gas bearing which comprises a relatively rigid
shaft and a plurality of relatively flexible foils for supporting
the shaft in operation on a cushion of air, wherein the shaft is
provided with a surface layer of a glaze-forming oxide between
0.003 ins. and 0.020 ins. thick, and each foil is provided with a
surface layer of a glaze-forming oxide between 0.0001 ins. and
0.0005 ins. thick.
Description
The present invention relates to surface coatings for machine
elements, of the kind in which there is relative rotation between a
relatively rigid component and a relatively flexible component at
high speed, for example foil gas bearings.
In the operation of foil gas bearings there is contact between the
two components at start-up and run-down of the bearing, and there
is also a possibility that under high loading there may also be
contact between the components at high speed.
In order to preserve the lives of the bearing surfaces of the
components they have hitherto been coated with low friction
materials such as P.T.F.E., but these materials have not been able
to operate at high temperatures such as, for example are found in
gas turbine engines. Thus in order to produce a bearing suitable
for use in a hot gas turbine environment it has been necessary to
develop a coating which has both the required low friction
properties and which does not break down at temperatures up to
550.degree. C.
It is known from published research results that oxides of certain
metallic elements form glazes during wear at elevated temperatures,
and that the glazes have low friction characteristics. However the
known results provide no information as to the application of such
glazes to the reduction of wear between relatively rotating
components of machine elements of the kind described.
We have now carried out research into applying glaze-forming
coatings to the components of such machine elements, in particular
with foil gas bearings, and have found that oxide layers above
1.001- 0.003 ins. thickness will not adhere to the flexible
components under operating conditions for a significant length of
time but will break up due to the flexing of the component. But if
the oxide layers are reduced in thickness they are more prone to
failure due to foreign object damage, for example if dust is
ingested into the machine element.
The present invention provides a machine element having a
combination of surface layers of glaze-forming oxides of different
thicknesses on its components which overcomes the above
problem.
According to the present invention in a machine element of the kind
described, the relatively rigid component is provided with a
surface layer of a glaze-forming oxide of between 0.003- 0.020 ins.
thick and the relatively flexible component is provided with a
surface layer of a glaze-forming oxide between 0.0001 ins. and
0.0005 ins. thick.
We have found that with the above combination of coatings on the
components of the machine element, the oxide will not crack off
from the flexible component, and although foreign object ingestion
will damage the thin oxide layer of the flexible component, even to
the extent of exposing bare metal, the thick oxide layer on the
rigid member will not normally be penetrated to expose bare metal,
and failure of the element is prevented. We have also found that
the thin glaze layer has self-healing properties in so far as the
exposed bare metal will oxidise, and rubbing contact will re-form
the glaze.
The thickness of the oxide layer on the relatively rigid component
is preferably in the range 0.003 ins. to 0.007 ins., and the
thickness of the oxide layer on the flexible component is
preferably of the order of 0.0002 ins.
The metallic elements from which the oxide layers are formed may be
chosen from nickel, chromium, iron and cobalt although it is
believed that other oxides may form glazes and any glaze-forming
oxide would be suitable.
According to a feature of the present invention, a foil gas bearing
comprises a relatively rigid shaft surrounded by a plurality of
foil elements, and the shaft is coated with a glaze-forming oxide
to a thickness in the range 0.003 to 0.007 ins, and the foils are
coated with a layer of a compound of cobalt and chromium carbide
the surface of which is oxidised to a depth of up to 0.0005
ins.
The invention will now be more particularly described with
reference to the accompanying drawings in which:
FIG. 1 illustrates a foil bearing to which the invention is
applied,
FIG. 2 is a cross-section of the bearing of FIG. 1 showing
diagrammatically the disposition of the foils and coatings, and
FIG. 3 shows a foil illustrating the oxided surface thereof.
Referring now to the drawings, a foil element bearing is
illustrated, and comprises a shaft 2 which is supported for
rotation in a bearing bush 4. The bush 4 is stationary and has
mounted in its interior, a plurality of foil bearing elements 6. In
the example shown, there are eight elements 6 each of which subtend
an angle of approximately 90.degree. and which overlap by about 50%
the adjacent elements. The elements 6 are held in place in slots in
the bush 4. Foil gas bearings are in themselves known, for example,
from U.S. Pat. No. 3,215,480, and since the present invention
relates to the treatment of the bearing surfaces of the components,
of this and other forms of machine elements, the bearing is not
described in great detail.
The coating on the shaft is depicted by the thick line 8 in FIG. 2
and the oxided surface layer of a foil is shown at 10 in FIG.
3.
In the operation of such a bearing the shaft rotates within the
foils 6, and air is trapped between the foils and the shaft so that
an air cushion is produced between the foils and a bearing surface
on the shaft, which cushion supports a load on the bearing by
preventing contact between the bearing surfaces.
The above described bearing is one example of a gas bearing which
runs without the usual oil lubrication, and such bearings are
usually required to run dry with the bearing surfaces in contact on
start up, for a sufficient length of time for the air cushion to
become established, and for the shaft to lift off the surface of
the bearing bush. During this time, the bearing surfaces wear due
to friction and very smooth wear resistant surfaces are
required.
The present invention provides a surface treatment which is
particularly applicable to gas bearings and which produces a very
smooth surface having low friction and wear properties.
Tests were conducted simulating a foil element bearing as described
with reference to FIGS. 1 and 2 but using a single foil pivotally
mounted above a shaft so as to be capable of being brought into
rubbing contact with a specially provided bearing surface on the
shaft, at varying bearing pressures. The shaft was driven during
the tests at a speed of approximately 5,500 r.p.m. and a series of
stopstart cycles was performed to represent the initial running
period of an air bearing, each test being terminated when "pick-up"
occurred.
Several combinations of coatings of oxides were tried and found not
to give the bearing any significant life. For example, it was found
that, if both the shaft and the foils were coated with 0.001 to
0.002 ins. of a material containing chromium or cobalt in
oxidisable quantitites and the surface layers of the coating were
oxidised to a depth of 0.0002 ins. by heating in the air, the
bearing was prone to failure due to ingestion of atmospheric dust
particles which scored the oxide layers and exposed the bare metal
substrates of the shaft and foils. The subsequent metal to metal
contact caused failure of the bearing.
Thick coatings of oxide on the foil, however, were found to break
up as the foil flexed.
The solution to these problems was found to be the provision on the
shaft of a coating of oxide of between 0.003 ins. and 0.007 ins.
thick, in combination with an oxide coating on the foil of between
0.0001 ins. and 0.0005 ins.
By this means the oxide layer on the foil was thin enough to flex
with the foil in operation without cracking, while metal to metal
contact after debris ingestion was avoided because the scoring of
the oxide surface on the shaft was not deep enough to expose
substrate material of the shaft to the scored foil. In addition it
was found that the thin oxide layer on the foil had a self-healing
property and the scores soon became smoothed out into a continuous
oxide surface again.
The thick oxide coating on the shaft was provided by plasma
spraying cobalt oxide or chromium oxide directly onto the shaft.
Although the cobalt oxide seemed to provide the better glaze, there
were difficulties in spraying the oxide, but the chromium oxide
could be applied by a commercially available process. Clearly other
oxides known to form glazes could be substituted. The thickness of
the coatings on the shaft may be varied but it is expected that a
minimum thickness of 0.003 ins. is necessary to avoid scoring
through the coating by dust and grit normally in the bearing
environment. In particular, in a gas turbine engine environment it
is believed that a coating thickness of up to 0.010 ins. is
required to give an adequate safety margin. Coatings of up to 0.020
ins. may be used on the shaft but at this thickness some cracking
was noticed after a relatively short life.
On the foil, however, it was not found to be possible to plasma
spray the oxide directly, although vapour deposition could be
used.
The preferred method was to first coat the foil with a compound
comprising 75% cobalt and 25% chromium carbide by volume to a
thickness of between 0.001 and 0.002 ins. and then to oxidise the
surface layer to a depth of approximately 0.0002 ins. by heating in
air. The cobalt and chromium carbide compound is sold under the
trade name of TRIBOMET 104C by the Bristol Aerojet Company.
The preparation of the bearing surfaces was important for producing
the required surface finish and the method found to produce the
best results was as follows:
On the shaft, the shaft surface was vapour blasted and sprayed with
a bond coat of Nickel Aluminide to a thickness of 0.002 in. to
0.003 in. and then Cobalt Oxide or chromium oxide was plasma
sprayed onto the surface to a depth of up to 0.010 ins. The surface
of the shaft was ground and lapped to a smooth finish. A similar
treatment would apply to other oxide coatings.
The foil, which was made from a Nickel-based alloy known as NIMONIC
90 and was 0.005 ins. thick was first acid etched and then
electro-plated with TRIBOMET T104C to a depth of 0.001 to 0.0015
ins. The surface layer was then diamond lapped to a thickness of
0.0005 to 0.001 ins. to a surface finish 1 to 2 micro ins. CLA
(Centre line average). This is a mirror finish. While a minimum
thickness of say 0.002 ins. of TRIBOMET 104C is preferable,
thicknesses of around 40% of the thickness of the foil may be used.
The lapping ensured that the variation in thickness of the foil
substrate and coating was uniform to 0.0002 to 0.0004 ins.
The coated foil was then oxidised in air by heating to 600.degree.
for 4 hours which gave a uniform oxide coating 0.0002 ins.
thick.
The invention has been described in relation to a particular type
of known air bearing. Clearly, however, the surface treatment of
the present invention is applicable to any machine element of the
kind described for example, seals which have relatively rigid and
relatively flexible components running at very small
clearances.
One of the main advantages of the surface treatment of the
invention is that the oxide coating has a high melting point, and
that enables bearings with the oxided surfaces to run at much
higher temperatures than hitherto. For example conventional gas
turbine engine bearings are limited at present to operation below
250.degree. C because of the temperature limitations on the
lubricating oil required. Air bearings with oxided surfaces can be
run at temperatures in excess of 550.degree. C and are limited more
by the substrate material than the oxide coating. In fact, the self
healing properties of the oxide glaze are improved as the
temperature increases.
* * * * *